On Mon, 2004-07-26 at 04:50, Ingo Molnar wrote:> * Lee Revell <rlrevell@joe-job.com> wrote:> > > Latency with 2.6.8-rc2 + voluntary-preempt-I4 is the best so far. > > After extended testing there only seem to be a few hot spots. In> > several minutes I saw an 11ms violation, a 14ms violation, and several> > 2ms violations.> > > > get_user_pages() is much better in 2.6.8-rc1-mm1 than 2.6.8-rc2. Is> > there any chance of getting the fix into mainline?> > in -J3 i've added a cond_resched to the latency-generating point of> get_user_pages(). (The biggest latencies happen via> make_pages_present(), which gets triggered by mlockall()/MAP_LOCKED.)>

OK, great, this is the biggest remaing issue with -I4, because jackddoes that a lot.

> > 2ms non-preemptible critical section violated 1 ms preempt threshold> > starting at unmap_vmas+0x1ff/0x210 and ending at> > unmap_vmas+0x1f5/0x210> > this is the normal sys_exit()->exit_mmap()->unmap_vmas() path. It's> weird that it generated a 2ms latency. What are the values of> voluntary_preemption and kernel_preemption on your current kernel? With> a 2:0 setting we ought to have a reschedule point every 32 pages. Do> you know which application triggers this latency and is it easy to> reproduce?>

2 and 1. Now that I think about it, this could have happened duringbootup, before my rc.local set these. I will try passing them on thekernel command line.

Not sure I understand the difference between 2:1 and 2:0. Would thelatter make the kernel only preemptible at the voluntary preemptionpoints?

> > 14ms non-preemptible critical section violated 1 ms preempt threshold > > starting at tty_write+0x1b6/0x290 and ending at schedule+0x2fd/0x5b0> > does this one trigger when you are using the VGA console? (or fbcon)?> > it's not immediately obvious to me precisely where this latency comes> from, it would be nice to know how to reproduce it.

It think this one was caused by switching virtual consoles. At onepoint Andrew Morton suggested I remove the (un)lock_kernel calls fromdo_tty_write. This fixed the problem, with no detectable side effects. Maybe this could be incorporated into voluntary-preempt, it would beuseful to have more than one person to test it.